I think it’s maybe due to the fact that the LNAV-only missed approach starts at the runway threshold, whereas the LNAV/VNAV missed approach starts earlier; at the DA/DH or MDA/MDH. Therefore the LNAV/VNAV missed approach segment encompasses a wider area for obstacle clearance (compared to the LNAV-only approach area) and there maybe an obstacle in the LNAV/VNAV missed approach footprint that falls outside the LNAV-only approach footprint.
In other words, there may be some obstacles which must be considered for the LNAV/VNAV approach and missed approach which may be ignored for the LNAV-only approach and missed approach, due to the different lateral dimensions of their obstacle consideration envelopes.